The present invention relates to a method for controlling at least one LED strip comprising a multiplicity of light-emitting diodes (LEDs) connected in series, wherein each of the LEDs is separately connected by a switching element and a controller connects the switching elements for connecting the LEDs by outputting drive pulses. The brightness generated by the LEDs is adjusted by means of pulse-width modulation (PWM) by changing the ON period of an LED within a drive frame, wherein the LEDs connected in series are driven temporally offset to one another. Such LED strips are used among other things for external illumination in headlights for automobiles and for signaling purposes (e.g., as a brake light).
Driving devices currently use pulse-pause modulation to vary the brightness of different LEDs which are connected in series. To this end, a switch arranged in parallel to the LED conducts a constant flowing current in the off-time, in which the LEDs do not output any light. To this end, in relation to a so-called matrix arrangement of LEDs, it is possible to operate all LEDs connected in series at their maximum illumination.
Furthermore, all LEDs of a so-called LED strip (LEDs connected in series) are driven temporally offset so that the chain-length or strip-length of the driven LEDs remains as constant as possible and the forward voltage is thereby subjected to a fluctuation as small as possible. This results in a kind of “block control” for an LED strip.
If the on-time of an LED now changes, all of the on-times of the LEDs driven in the block (or strip) will shift. Within one clock pulse (drive frame), which has a temporal length of 5 ms for example, the LEDs are often driven only for a specific duration within the clock pulse. In normal operation, certain intervals thereby arise between the states in which the LEDs are connected luminous. If, for example, an LED is connected for 2 ms at the start of a clock pulse (drive frame) which has a temporal length of 5 ms, then a time period of 3 ms will pass until this LED is again connected at the start of the next clock pulse (drive frame).
In the change of on-time given above, at least one LED will nevertheless experience a discontinuous shift in on-time from other LEDs at the time of the change, which, in an extreme case, will in turn result in a gap or a double drive. In the example given above, a shift of the on-time and therefore of the drive pulse, by 3 ms for example, would mean that prior to the change the LED is normally connected within the clock pulse (drive frame) at the start of the clock pulse (drive frame), and that, after the change of the on-time (shift), the LED is connected within the clock pulse for 2 ms only at the end of this clock pulse (drive frame), wherein the LED would thus be disconnected for 6 ms, thereby resulting in a visible (light) gap. But there can also occur a shift in which the LED is connected for 2 ms at the end of the clock pulse (drive frame) before the shift (change) and connected for 2 ms at the start of the next clock pulse (drive frame) after the shift, thereby resulting in a total ON period of 4 ms (double drive).
Such an effect arises in particular at the moment that the on-time of an LED is changed, wherein after this change, i.e. after the shifting of the on-times of the LEDs, they are again operated with the same temporal intervals. But during the change of the on-time, the human eye can perceive this as flickering. By means of frequency analysis, it can be determined that the gap or double drive results in a spectrum which exhibits frequencies in the range under 100 Hz and corresponding amplitudes.
This flickering is disturbing however and, in the case of headlights of motor vehicles for example, can disturb other traffic participants.